Adipose tissue growth and development can be described as an orderly sequence in which cell number is developed early in life and is normally maintained constant thereafter. Cell size on the other hand remains more flexible. An "adipose cell hypothesis" has been elaborated which states that obesity can be a disorder of both cell number development as well as hypothesized controls linking cell size and energy metabolism. Various aspects of this hypothesis are being tested in animals. The techniques of lipectomy, or surgical removal of fat, and diet-induced adipocyte hyperplasia performed at various times in development are being used as tools to examine the proliferative capacity of adipose tissue as it changes from early life to adulthood. Adipose tissue cellularity and studies of DNA turnover in adipocytes have been essential additions to these studies to permit a full and critical evaluation of the results. Both fat cell size and number may influence feeding behavior and metabolism in the rat, but most likely in different ways. Since any abnormality of such influences would be of consequence to the body weight and feeding behavior of the rat (i.e. may cause the rat to become obese), efforts to elaborate their nature will continue as a primary endeavor in this laboratory.